Die separation, or dicing, by sawing is the process of cutting a microelectronic substrate into its individual circuit die with a rotating circular abrasive saw blade. This process has proven to be the most efficient and economical method in use today. It provides versatility in selection of depth and width (kerf) of cut, as well as selection of surface finish, and can be used to saw either partially or completely through a wafer or substrate.
Wafer dicing technology has progressed rapidly, and dicing is now a mandatory procedure in most front-end semiconductor packaging operations. It is used extensively for separation of die on silicon integrated circuit wafers.
Increasing use of microelectronic technology in microwave and hybrid circuits, memories, computers, defense and medical electronics has created an array of new and difficult problems for the industry. More expensive and exotic materials, such as sapphire, garnet, alumina, ceramic, glass, quartz, ferrite, and other hard, brittle substrates, are being used. They are often combined to produce multiple layers of dissimilar materials, thus adding further to the dicing problems. The high cost of these substrates, together with the value of the circuits fabricated on them, makes it difficult to accept anything less than high yield at the die-separation phase.
Dicing is the mechanical process of machining with abrasive particles. It is assumed that this process mechanism is similar to creep grinding. As such, a similarity may be found in material removal behavior between dicing and grinding. The theory of brittle material grinding predicts linear proportionality between material removal rate and power input to the grinding wheel. The size of the dicing blades used for die separation, however, makes the process unique. Typically, the blade thickness ranges from 0.6 mils to 50 mils (0.015 mm to 1.27 mm), and diamond particles (the hardest known material) are used as the abrasive material ingredient. Because of the diamond dicing blade's extreme fineness, compliance with a strict set of parameters is imperative, and even the slightest deviation from the norm could result in complete failure.
FIG. 1 is an isometric view of a semiconductor wafer 100 during the fabrication of semiconductor devices. A conventional semiconductor wafer 100 may have a plurality of chips, or dies, 100a, 100b, . . . formed on its top surface. In order to separate the chips 100a, 100b, . . . from one another and the wafer 100, a series of orthogonal lines or "streets" 102, 104 are cut into the wafer 100. This process is also known as dicing the wafer.
Dicing saw blades are made in the form of an annular disc that is either clamped between the flanges of a hub or built on a hub that accurately positions the thin flexible saw blade. As mentioned above, the saw blade employs a fine powder of diamond particles that are held entrapped in the saw blade as the hard agent for cutting semiconductor wafers. The blade is rotated by an integrated DC spindle-motor to cut into the semiconductor material.
Optimizing the cut quality and reducing process variation requires an understanding of the interaction between the dicing tool and the material (substrate) to be cut. The most accepted model for material removal by abrasion is described in Wear Mechanisms in Ceramics, A. G Evans and D. B Marshal, ASME Press 1981, pp. 439-452. This model predicts the threshold load that must be applied by the abrasive grain to cause fracture of the brittle ceramic. The cracks create localized fracture in the material in predicted directions. Material is removed as particles when some of the cracks join in three dimensions. The Evans and Marshall model predicts the linear relation between the volume of material removed by an abrasive particle and the load exerted by this particle according to the following equation. ##EQU1##
where, V is the volume of material removed, Pn is the Peak Normal Load, .alpha. is a material independent constant, K is a material constant, and 1 is the cut length. The value of .alpha./K is in the range of 0.1 to 1.0.
Assuming formula reciprocity, it follows that the measured load should have a linear relationship to the material removed. In other words, if a known volume of material is removed, then the abrasive cutting wheel has exerted a known load on the substrate.
According to Grinding Technology, S. Malkin, Ellis Horwood Ltd., 1989, pp. 129-139, a high percentage of mechanical energy input turns into heat during the abrasive process. Excessive heat generation due to friction, which may be observed as deviation from the linear relationship between material removal and load, can cause damage to the workpiece and/or dicing blade, possibly resulting in destruction of one or both.
Prior art systems for monitoring dicing operations rely on visual means for determining the quality of the cut in the substrate. These prior art systems have the drawback that the cutting process must be interrupted in order to visually inspect the kerfs. Furthermore, only short sections of the cut are evaluated in order to avoid the excessive time requirements for a 100% inspection. The results of the short section inspection must be extrapolated in order to provide full evaluation. In addition, these visual systems only allow for the inspection of the top surface even though the bottom surface is also subject to chipping. Therefore, evaluation of the bottom of the semiconductor wafer must be performed off-line. That is, by stopping the process and removing the wafer from the dicing saw to inspect the bottom surface of the wafer.
There is a need to monitor blade load during wafer or substrate dicing for optimizing the dicing process and maintaining a high cut quality so as not to damage the substrate, often containing electronic chips valued in the many thousands of dollars. There is also a need to perform monitoring over the entire length of the cut and to avoid the need for interrupting the process during the monitoring.